Any app distribution platform carries with it a risk of hosting user-hostile software, so it is certainly not immune. Fortunately there are fewer incentives for trying to sneak such software into F-Droid which results in fewer (if any) straight up lies intended to trick users into buying misleading software or exposing their data for the sake of ad revenue.
This is of course the same issue we all face when opting to pull in an OSS dependency for our own projects (from npm or docker or rubygems or rust crates or...): we need to decide on our own how far to trust the software maintainers.
The android permissions model offers some degree of protection in both stores from hostile software. However, unlike the Play Store which offers only a couple of tags (contains in-app purchases / contains ads), in F-Droid any known "antifeatures" (i.e. association with paid services) are listed explicitly in the catalogue.
As for abandonware (or other software that F-Droid drops for practical concerns), users could still acquire the code and build it themselves, which means that a developer stepping away from a piece of software does not mean a user needs to say goodbye.